Advice for cobra IRS swap?

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Warhorse Racing

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You will need to figure out how to get the rear brake lines to work. MM makes a kit (MMBAK-21) to adapt the IRS brake lines to earlier cars. Double-check that it will work with your SVO.

I have purchased 3 "low mileage" IRS assemblies over the years, every one needed the differential rebuilt. I would recommend checking the diff before you install it in the car.

If you are planning to use the IRS for performance driving, I highly recommend upgrading to Full Tilt Boogie Racing bushings (and their other IRS improvements). The OEM IRS bushings are not very good. The Cross-Axis Joint Boots also tend to go; it's worth replacing those, too.

I opted to go with poly UCA and LCA bushings on my IRS-swapped 2004 V6 and wheel hop hasn't been an issue (220 hp). There is more wheel hop with the poly bushings on my IRS-swapped 2000 GT (300 hp, more tq than my V6). If you are planning to add a ton of power, you might want to go with the FTBR Delrin UCA/LCA bushings.

Some other issues you will encounter:

1. You will need to modify the frame where the quad shocks mount to get the Rear IRS Brackets to mount correctly.
2. You will need to spread the lower torque boxes to get the IRS subframe to fit properly (the FTBR Bushing Removal Tool helps with this process).
3. Make sure you remove the pinion snubber on your chassis.
4. Install the sway bar and the UCAs before installing the IRS subframe into place.
5. You cannot jack the car up from any part of the IRS assembly. Many people/shops don't know this and they end up damaging the IRS.
6. It is possible that you will have to shorten your driveshaft, but I'm not 100% sure about the SVO driveshaft length. I had to take 1" out of the driveshafts on both my New Edge cars.

I have videos on my YouTube channel about the parts I used and the issues I encountered on my 2004 V6, most of which will pertain to any IRS-swap. I can post them in this thread if you'd like.
 

ClayBelt

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SVO has a 7.5” diff like the V6, that’s a good point I hadn’t thought about. I guess I’ll need to track down a V8 driveshaft or have mine shortened. Thinking Prothene bushings and SPC cross axis joints if I need them, and I’ll probably fab my own brake lines to split off th existing ones.
 

Warhorse Racing

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SVO has a 7.5” diff like the V6, that’s a good point I hadn’t thought about. I guess I’ll need to track down a V8 driveshaft or have mine shortened. Thinking Prothene bushings and SPC cross axis joints if I need them, and I’ll probably fab my own brake lines to split off th existing ones.
The SVO has some unicorn parts, so please double check this...

I was able to change the pinion flange on my 03-04 Cobra IRS differential to the 79-04 GT 8.8 pinion flange. That allowed my Ford Racing Aluminum driveshaft to match up with the differential. I think (but I'm not 100% sure) that the 99-01 Cobras had the smaller pinion flange.

If you look at the Ford Racing M-4602-G aluminum driveshaft, it fits:
  • 79-95 Mustang 5.0L/5.8L with Manual Transmission (SROD/T5)
  • 79-95 Mustang 5.0L/5.8L with Manual Transmission (Tremec 3550 - Must use driveshaft spacer on 94-95)
  • 83-95 Mustang 5.0L/5.8L with Automatic Transmission (C4/AOD)
  • 94-04 Mustang 3.8L/3.9L V6 with Manual Transmission (T5)
  • 96-04 Mustang 3.8L/3.9L V6 with Automatic Transmission (Must Re-use Factory Transmission Yoke from Stock Driveshaft)
  • 1984-1986 SVO
I bolded the fact that it fits both 7.5 and 8.8 SRA cars. That leads me to believe that your driveshaft will bolt up to the smaller pinion flange used on non-IRS 8.8 cars. It might be wise to try to mate your SVO driveshaft up to the '01 IRS pinion flange you have. It might fit. If not, you need to swap out the pinion flange on the IRS.

Do NOT use poly in the IRS subframe or the differential bushings; poly deflects and you will get wheel hop. The FTBR UHMW Subframe bushings and their IRS differential bushings are well worth the money. It takes a TON of time to swap out the bushings in an IRS. You don't want to spend all that time and be disappointed. The FTBR parts are well worth the money.

Here are the videos that show my IRS mods, and offer some install tips, provide part #s, and reviews the results...

View: https://www.youtube.com/watch?v=9T46tdvDnt4


View: https://www.youtube.com/watch?v=g4Ezcsopi7M&t
 

ClayBelt

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It won’t just be a street car. This is intended to be a weekend toy and autocross car, with occasional mountain time. Will the delrin really make that big a difference compared to poly in a turbo 4 cylinder car that doesnt spool up until almost 3000rpm?
 

Warhorse Racing

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It won’t just be a street car. This is intended to be a weekend toy and autocross car, with occasional mountain time. Will the delrin really make that big a difference compared to poly in a turbo 4 cylinder car that doesnt spool up until almost 3000rpm?
I'm coming from a purely autocross perspective. If wheel hop costs me .300 of a second, I can lose. An SVO can really pull when it's in boost, and you will need to keep it in boost to avoid lag out on course. Add to that sticky autocross tires and you can get wheel hop when launching and digging out of slow/tight elements. Your car will be very similar to my 2004 V6, it has to handle better than cars that can outpower it on course.

I hope I'm helping you, but I would also recommend getting advice from people who have actually autocrossed with an all poly setup in their IRS. Personally, I wouldn't use anything but the FTBR bushings in the subframe and the differential. I have been fine using Prothane poly bushings in the UCAs and LCAs.

You're going to spend a lot of time doing the bushing swap. In my opinion, I'd rather have better bushings than I need, than need better bushings than I have.
 
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ClayBelt

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I wouldn’t even be considering anything else if I wasn’t doing this on a college kid’s budget tbh. I thought wheel hop was a launching issue only?
 

90sickfox

I didn't really have an issue with the stink...
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I had to shorten my driveshaft, FPP aluminum, to put the IRS in my 89 convertible, used it in my 84, and in my 88 notch. I recommend using a punch to mark the metal of the trunk after drilling through the bottom of the frame rail. You can use a piece of sharpened " all thread " bar from Lowes or HD. I found using a pre drilled 2×4 allows me to keep it straight and square to the hole from the bottom. Once you dent the top sheet metal drill a hole in the trunk and use a metal file bit to open it up. I traced a circle about 1 inch round ( just big enough for washer to fit through ). This way you can drop washers and bolts through the frame rail from the top with a magnet. The sheet metal is very thin up there. My first swap ( in 2006 ) I used plates of steel on the trunk floor to sandwich the rail. Now, I use the method detailed above and use rubber grommets to fill the holes.

My 04 IRS is street driven with stock bushings. No issues with wheel hop yet. Keep spare wheel bearing hubs on hand. I street drive and have replaced bearings a couple times. They tend to go bad without much warning. Haven't had an issue since I started torquing them to spec every once and a while. I haven't really driven the car either in the last 4 years.

E brake cables for all sn95 cars are the same...including IRS. I used the handle and transmission tunnel bracket from an sn95. Bracket is spot welded in. Had to modify the square bracket under the elbow rest for it to fit 1987+. If using sn95 e brake handle make sure the recall modification has been done.

Cut down a bolt to make a stud for the drivers side fuel tank strap. Its difficult to get a bolt started from the bottom with the IRS installed.
 

Warhorse Racing

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I wouldn’t even be considering anything else if I wasn’t doing this on a college kid’s budget tbh. I thought wheel hop was a launching issue only?
I totally understand being on a budget. At the very least, consider using the FTBR bushings for the differential. That's the most important area. Remember, there are bushings on the front and back of the pumpkin and they both need to be upgraded at the same time.

Wheel hop happens most often when launching. But, when you have to slow down in tight elements, you can get wheel hop when the power comes in as you dig out of them. If the turbo hits at the wrong time, you can get wheel hop in those situations. You are essentially launching out of a slow element (like a pivot cone).

The level of aggression when driving at an autocross is much higher than what can (and should) be done on the street. That exposes the weak links in the suspension.
 
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ClayBelt

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So here’s my thought process then, based on this new to me information: FTBR subframe and diff bushings, with stock rubber where needed in the control arms. Does that sound like a bad plan for a budget weekend street car with occasional autocross or mountain driving?
 

Warhorse Racing

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So here’s my thought process then, based on this new to me information: FTBR subframe and diff bushings, with stock rubber where needed in the control arms. Does that sound like a bad plan for a budget weekend street car with occasional autocross or mountain driving?
I think that's wise. You can swap out the LCA/UCA bushings without having to completely remove the IRS subframe, so it makes sense to do the other bushings first.

If you find that your subframe and diff bushings are deteriorated (which happened on 2 of my IRS assemblies), you should check the rubber UCA/LCA bushings, as they might be bad too.

An IRS-swapped SVO is one of my dream autocross builds. I hope you autocross that car as much as possible.
 
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90sickfox

I didn't really have an issue with the stink...
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Kenny Brown used to have a video of all the deflection an IRS goes through during aggressive driving. They used to make control arms for IRS years ago. My cars hardly ever see track time. Occasional burn outs and hard pulls.

Several years ago I had a couple customers with what they called wheel hop. The issue was that the driveshafts were too long and causing a vibration. The front bushings on the differential were also worn on both of them. On my current IRS I have shims made from washers to tighten up the front bushings. Its somewhere in my build thread. I have tracked this IRS in the past and noticed a little snap out of hard corners. ( always thought that was because of the stock control arm bushings ). The most power I've seen is about 300hp at the tires. My current set up should be at least 400hp at the tire ( turbo 331 9psi. ). I haven't gotten on it enough to know if I'll start having IRS hop issues.

One must is aftermarket tie rods. Mine have bent several times over the years. I've been cheap and using stock ones from customers upgraded units. This time I'm ordering good ones. Mine bent on a pull to full boost while data logging . In the past I can't pin point when they bent. Just noticed while wrenching or when checking the alignment. This time they are bent significantly more.
 

Warhorse Racing

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Feb 10, 2019
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Kenny Brown used to have a video of all the deflection an IRS goes through during aggressive driving. They used to make control arms for IRS years ago. My cars hardly ever see track time. Occasional burn outs and hard pulls.

Several years ago I had a couple customers with what they called wheel hop. The issue was that the driveshafts were too long and causing a vibration. The front bushings on the differential were also worn on both of them. On my current IRS I have shims made from washers to tighten up the front bushings. Its somewhere in my build thread. I have tracked this IRS in the past and noticed a little snap out of hard corners. ( always thought that was because of the stock control arm bushings ). The most power I've seen is about 300hp at the tires. My current set up should be at least 400hp at the tire ( turbo 331 9psi. ). I haven't gotten on it enough to know if I'll start having IRS hop issues.

One must is aftermarket tie rods. Mine have bent several times over the years. I've been cheap and using stock ones from customers upgraded units. This time I'm ordering good ones. Mine bent on a pull to full boost while data logging . In the past I can't pin point when they bent. Just noticed while wrenching or when checking the alignment. This time they are bent significantly more.
If I dump the clutch on a launch in my IRS-swapped GT (300 hp), I will get wheel hop. And it can be pretty harsh. Sometimes what's causing wheel hop, even with the best bushings, is the driver. At an autocross, you never want to launch too hard; spinning your tires means losing time. So, there's always a balance of how hard to launch. And, with a stiff chassis and grippy tires, a hard launch is going to lead to either wheel spin or wheel hop. Every car is different. You want a setup that allows for a launch with the most amount of controlled aggression.

Having been in autocross Mustangs with the stock IRS bushings, I can say for sure that improving the bushings reduces wheel hop.

In a car with less low-end torque and less horsepower, like the OP's SVO and my V6, you have to enter elements faster and get on the gas earlier to keep your speed up. You have no power below 2,500 rpm, so momentum is your best friend. That means you have to handle better than other cars to get through elements.
 

eighty5fox

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I recently did this swap and did not need to run a shorter driveshaft. That was with a tremec 3550 i believe it is the same length as a t5. You also may want to check your pinion flange the irs and the solid axle use different flanges. My irs is the 01 and it used the correct flange and the 03/04 cobra flange is different. I upgraded all bushings with poly energy suspension bushings that were cheap and perform perfect every where i take the car i have put at least 15k miles on my irs swap and have no complaints.
 
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